1. Field of the Invention
The present invention relates to an optical encoder and a scale for an encoder, and in particular, to a scale in which optically anisotropic regions, which vary the polarization state of incident laser light, are distributed, and to an optical encoder which detects an amount of movement by using the scale.
2. Description of the Related Art
A conventional optical encoder is formed so as to include a light source which outputs laser light, a scale which has a diffraction grating and is movable in a predetermined direction, a collimator lens which makes the laser light into parallel light and makes the parallel light incident on the diffraction grating of the scale, a photo-detector which detects changes in the intensity of the diffracted light at a predetermined position, and the like. In this optical encoder, the diffracted light intensity detected at a predetermined position varies periodically due to movement of the scale. This periodic variation can be associated with the pitch of the diffraction grating. Thus, the amount of movement of the scale (the amount of displacement from a reference point) can be detected from the period of the variation and the pitch of the diffraction grating.
For example, Japanese Patent Application Laid-Open (JP-A) No. 62-200224 discloses the optical encoder shown in FIG. 28. In this optical encoder, light emitted from a laser 1 is made into parallel light at a collimator lens 2, is transmitted through a beam splitter 3, and is incident on a diffraction grating 4 which moves. The lights which are diffracted by the diffraction grating 4 are reflected at mirrors 5, 5′, and are again superposed at the beam splitter 3, and this light is detected at photo-detector 6. The amount of movement of the diffraction grating 4 can be known in accordance with the variation in intensity. However, in the structure shown in FIG. 28, the beam splitter, the lens, the mirrors and the like must be arranged spatially with very high precision. Accordingly, manufacturing is difficult, the size becomes larger of necessity, and costs increase.
Further, for example, JP-A No. 7-306058 discloses the optical encoder shown in FIG. 29. In this optical encoder, a surface-emitting laser is used as the light source in order to make the device more compact. Because the exiting angle of the beam from the surface-emitting laser is small, no collimator lens is needed, and the device can be made more compact. The beam emitted from a surface-emitting laser 41 is reflected at a linear scale 42, and the reflected light is received at light receiving elements of a photo-detector 431. Further, the beam emitted from the surface-emitting laser 41 is transmitted through the linear scale 42, and the transmitted light is received at photo-detecting elements of a photo-detector 432. Note that, in FIG. 29, the structure functions as an encoder provided that there is at least either one of the photo-detector 431, which detects the light reflected from the scale, and the photo-detector 432, which detects the light transmitted through the scale. In accordance with this structure, as compared with the optical encoder disclosed in JP-A 62-200224, the number of parts can be reduced, and the device can be made more compact.
However, in a conventional optical encoder in which the principle of movement detection is the varying of the laser light intensity due to movement of the scale, there is the serious problem that the detected light intensity greatly fluctuates due to fluctuations in the output light intensity of the laser light source, external light, positional offset between the scale and the detector, and the like, and the exact amount of movement cannot be detected. For example, if the distance between the scale and the detector is large, the signal intensity detected at the detector decreases, and the exact amount of movement cannot be detected. Thus, a conventional optical encoder has many restrictions from the standpoint of design, such as highly-precise alignment and an appropriately light-shielded environment are required, and the like, and can only be used in limited applications and environments.
Further, because the amount of movement is measured in accordance with the variation in the laser light intensity, it is difficult to confirm the direction of movement of the scale.